1. Field of the Invention
The present invention relates to a turbine rotor and a turbine blade having an inverted fir tree blade root for insertion in an axial direction of the rotor.
2. Description of the Related Art
The adoption of longer blades in the low-pressure last stage of a steam turbine is increasing to ensure higher capacities and efficiency of the steam turbine. Accordingly, blade grooves tend to be dimensionally extended so that the stresses applied to the blade grooves will be reduced with respect to increased centrifugal stress due to the adoption of longer blades.
However, since the dimensional extension of the blade grooves correspondingly increases the radial depth of rotor grooves, the rotor becomes difficult to machine and as a result, machining jigs for cutting the rotor grooves are required to be highly rigid.
In particular, if the circumferential width of the innermost circumferential rotor hook is not large enough, it is necessary to use a grooving cutter whose lowest section is flaccid and flexible.
These conditions are likely to break the grooving cutter during rotor cutting, and thus to make the rotor non-usable, and the flexure of the grooving cutter is likely to result in the contact section of the rotor hook being cut too much and render the rotor hook unable to bear a required load. Accordingly, reliability is likely to be adversely affected.
For a turbine rotor having an inverted fir tree blade root, therefore, there is a need to prevent a grooving cutter from being broken during rotor cutting, by increasing the circumferential width of the innermost circumferential rotor hook.
A technique for preventing a grooving cutter from being broken is described in JP-B-07-72485, for example. Patent Document 1 discloses a structure in which: a hook on the innermost surface of a rotor is formed with a large circumferential hook width with respect to the circumferential width of a neck formed on the innermost surface of a blade, and a space is formed between the innermost opposed surfaces of the blade neck and the rotor hook.
In addition to the above, known analogous techniques are described in Japanese Patent Publication No. 2877150 and JP-A-05-86805.